In continuation of an earlier paper on Pleurosigma species with an (almost) non-sigmoid valve and raphe sternum, a comparative study was made in LM and SEM of Pleurosigma subrectum and P. acus. For P. subrectum, slides and a subsample of the type material were examined. For P. acus no unmounted material permitting SEM investigation is extant; a sample containing specimens fully matching the type in LM was therefore used as epitype material for SEM. The original data on striation of P. acus are emended. No morphological differences indicating separate specific status of these two taxa were observed and P. acus is therefore here designated a heterotypic synonym of P. subrectum. From the data now available, this is a very widely distributed species. The study demonstrates the indispensable role of collections for investigations on the diversity and distribution of diatom species.

A relational database linking benthic diatom records, taxonomic nomenclature including synonyms, and corresponding environmental data has been built in MS Access. It allowed flexible and long-term use of a relatively important amount of data (3000 records) gathered in the framework of the EC-funded PAEQANN project, gathering precise and documented information both about benthic diatoms and quantitative or semi-quantitative environmental data. Such a database has been shown to be a useful tool for the definition of benthic diatom typology at a multi-regional scale, the prediction of the impact of environmental characteristics on the structure of diatom communities, and additionally for a new insight on the auto-ecology of some taxa. This database could serve as a template for further work on diatoms and, after some implementation, on other freshwater communities. It could also be the basis for wider typology of stream diatoms, extended to other regions.

The marine araphid diatom Pteroncola pottercovei sp. nov. is described for a coastal Antarctic environment, based on light and scanning electron microscopy observations. The new species was found as solitary cells in the phytoplankton and epiphytic on the chains of other diatoms. It can be distinguished from P. inane, the only recognized species of the genus up to now, mainly by the presence of a well developed sternum and a hyaline mantle, apical pore fields composed of three horizontal rows of pores, a single rimoportula per valve, and a lower striae density. Two morphotypes could be distinguished in relation to the ultrastructure of alveolate (chambered) striae in external view. While most specimens presented striae formed by transapically elongated slits others showed striae composed of a row of small areolae. An emended description of Pteroncola is provided as well as a comparison with related genera in the Fragilariaceae, such as Brandinia, Catacombas, Hyalosynedra, Fragilaria, Fossula, Staurosirella and Thalassioneis.

The marine araphid diatom Pteroncola carlinii sp. nov. from a coastal Antarctic environment was described based on light and scanning electron microscopy observations. The new species was found as solitary cells in the phytoplankton and epiphytic on green algal filaments or diatom chains. It was distinguished from P. inane, the only recognized species, primarily by a well developed sternum, a hyaline mantle, apical pore fields composed of three horizontal rows of pores, a single rimoportula per valve, and a lower striae density. Two morphotypes of P. carlinii were distinguished based upon the ultrastructure of alveolate (chambered) striae in external view. While most specimens presented striae formed by transapically elongated slits, others showed striae composed of a row of small areolae. A revised description of Pteroncola was provided as well as a comparison with related genera in the Fragilariaceae, such as Brandinia, Catacombas, Fragilaria, Fossula, Hyalosynedra, Staurosirella and Thalassioneis.

A new diatom, Arcanodiscus platti gen. nov. et sp. nov., is described from the Argentinian Patagonia with a novel combination of features. Frustules are robust, strongly silicifi ed, with large and thick central hyaline area and thick valve mantle, under the valve edge often with external parallel undulations. Valves without labiate or strutted processes, the valve mantle areolae internally with otae; a combination of features that has not been described before. The new taxon is morphologically closely related to several representatives of the genus Melosira C.A. Agardh, e.g., M. robusta Hustedt (Houk comm.). However, it diff ers from them by the discoid frustules solitary or forming short fi laments, non-oculate, tubular areolae occluded internally by rotae, lacking of processes, but having a single special round apperture, herein termed the portula. Therefore, a new order and family, Arcanodiscales ord. nov. and Arcanodiscaceae fam. nov., are here described to allocate the genus Arcanodiscus gen. nov. Morphological details of the new species, based on a combined light (LM) and scanning electron (SEM) microscopy, as well as ecological aspects, are discussed in the light of available information.